Tracers and method of marking liquids

ABSTRACT

A method of marking a hydrocarbon fuel, the method comprising adding to said hydrocarbon fuel a tracer compound for marking the hydrocarbon fuel, the tracer compound being a substituted fluorene having a structure of Formula (I): wherein R1 and R2 are the same or different and selected from hydrogen, straight chain, branched or cyclic alkyl groups, phenyl or substituted phenyl groups, benzyl or substituted benzyl groups, or R1 and R2 form a single substituent linked intramolecularly to each other, or R1 and R2 are ether groups excluding acetal groups, wherein R3 and R4 are the same or different and selected from hydrogen, straight chain, branched or cyclic alkyl groups, phenyl or substituted phenyl groups, benzyl or substituted benzyl groups, and wherein at least one of R1, R2, R3, and R4 is not hydrogen.

FIELD

The present specification concerns marking liquids, especiallyhydrocarbon liquids, with tracer materials. The present specification inparticular concerns marking hydrocarbons which are taxable or liable tobe subject to tampering or substitution, such as gasoline and dieselfuels for example.

BACKGROUND

It is well-known to add tracers to hydrocarbon liquids. A typicalapplication is the tagging of hydrocarbon fuels in order to identify thefuel at a subsequent point in the supply chain. This may be done foroperational reasons, e.g. to assist in distinguishing one grade of fuelfrom another, or for other reasons, in particular to ensure fuelquality, deter and detect adulteration and to provide a means to checkthat the correct tax has been paid. Apart from fuels, other productssuch as vegetable oils or additive packs may be marked to identify theproduct is produced at a particular source or certified to a particularstandard.

One problem which is known to exist with the marking of fuel liquids inparticular is the potential for the tracer to be removed for unlawfulpurposes such as avoidance of paying tax, by evaporation from the fuel,by degradation of the tracer through ageing or exposure to environmentalconditions such as heat, sunlight, air or other methods of deliberateremoval. Methods for deliberate removal of tracers include adsorption ofthe tracer onto common adsorbent materials such as charcoal or clays,exposure to radiation, such as ultraviolet light, oxidation etc. Auseful fuel tracer therefore needs to be resistant to removal by thesecommon methods and also to treatment with acids and/or bases oroxidants. It is an aim of the invention to provide tracer compounds andmethods of marking hydrocarbon liquids which are more resistant toremoval of the tracer than other known tracers.

WO2012/125120, U.S. Pat. No. 6,808,542, and CA2365814 disclose the useof photoluminescent fluorene copolymers for marking fuels and otherproducts.

WO2018/182437 discloses a coating material for marking plasticscontaining a base of the coating material and fluorescent markers to aididentification of the plastics and sorting of plastic waste. The base ofthe coating material is disclosed as being lacquer, silicone or aqueousdispersion of resins. Numerous possibilities for the fluorescent markersare disclosed as options including fluorene.

WO2019/195013 discloses the use of xanthenes as fuel markers.

WO2019/195016 discloses the use of substituted dibenzofurans as fuelmarkers.

SUMMARY OF INVENTION

A method of marking a hydrocarbon fuel is provided, the methodcomprising adding to said hydrocarbon fuel a tracer compound for markingthe hydrocarbon fuel, the tracer compound being a substituted fluorenehaving a structure of Formula I:

-   -   wherein R1 and R2 are the same or different and selected from        hydrogen, straight chain, branched or cyclic alkyl groups,        phenyl or substituted phenyl groups, benzyl or substituted        benzyl groups, or R1 and R2 form a single substituent linked        intramolecularly to each other, or R1 and R2 are ether groups        excluding acetal groups,    -   wherein R3 and R4 are the same or different and selected from        hydrogen, straight chain, branched or cyclic alkyl groups,        phenyl or substituted phenyl groups, benzyl or substituted        benzyl groups, and    -   wherein at least one of R1, R2, R3, and R4 is not hydrogen.

The substituted fluorene tracer compounds as defined above have severaladvantages over prior art tracers as discussed below.

It has been found that fluorene is susceptible to adsorption byactivated charcoal, which is a common laundering agent as mentioned inthe background section. It is considered that this is because of π-πinteractions between the aromatic rings of the fluorene molecule and theactivated charcoal. Another factor contributing to fluorene's lack ofresistance to laundering is considered to be the presence of weaklyacidic protons at the C-9 position of the fluorene molecule.

To solve the problem of adsorption and increase the resistance tolaundering of tracer compounds based on fluorene, it is possible tomodify the aromatic rings of the fluorene molecule and/or the carbon atthe C-9 position, ideally with bulky non-planar groups. These bulkynon-planar groups inhibit the interaction of the molecule withadsorbents used in laundering. In doing so, these inherently non-polarmolecules additionally become non-planar. The net result is a family ofmolecules which is harder to launder from a hydrocarbon fuel or otherhydrocarbon liquid after treatment with acids, alkalis or repeated useof activated charcoal as well as other reagents used in fuel laundering.

It has also been found that while the additional functionalization offluorene molecules with non-planar groups increases their mass, themolecules are surprisingly quick-eluting by gas chromatography for theirmass. The combination of higher mass while remaining relatively quickeluting is a very useful combination of properties as it means thetracer molecules elute at least with some of the components of thehydrocarbon liquid in which they are disposed but can still be resolvedfrom those components by virtue of their mass. For example, the tracermolecules as described herein are heavier than most of the components ofa typical fuel (gasoline or diesel fuel) but are still readilydistinguishable from the fuel components which elute at a similar rateas the tracer molecules. By way of comparison, known non-polymeric,non-halogenated, non-launderable tracers typically have masses of lessthan 300 atomic mass units (amu). However, analysis of diesel fuels hasshown that there is a broad distribution of molecular weights between100-400 amu, particularly 100-300 amu, but increasingly few componentsheavier than 300 or 400 amu. Substituted fluorene molecules are readilysynthesised with molecular weights over 300, 350, 400, 450, or 500 amu.This makes the molecules readily distinguishable from components ofgasoline and diesel fuels. At the same time, the non-polymeric moleculesof the present invention are still sufficiently light (e.g. less than1000, 800, 600 amu) so as to have relatively fast elution times in GC-MSanalysis, unlike the fluorene copolymers mentioned in the prior art.

Furthermore, it is often the case that prior art tracer moleculesoperate best in one or other of gasoline and diesel but not both. Due tothe combination of properties as outlined above, the substitutedfluorene tracer molecules as described herein can be detected well inboth fuels while satisfying the other critical requirement ofnon-launderability. The tracer molecules can also be used for markingkerosene-based fuels, liquified petroleum gas fuels, bio-diesel fuels,or bio-ethanol fuels.

Further still, the tracer molecules of the present invention can consistof atoms selected only from the group carbon, hydrogen, and oxygen whichis a specified requirement for certain fuel marking applications.Additionally, the tracer molecules do not contain reactive functionalgroups or fused-ring structures which would otherwise decrease theirresistance to laundering.

Finally, the basic fluorene structure enables a family of related tracermolecules to be derived. That is, forming a substituted fluorene confersthe advantage that a suite of molecular tracers can be produced simplyby varying the species that is reacted with the fluorene core. The Rgroups of the fluorene, while typically being C₃ to C₂₀ groups, can beintentionally varied to provide a suite of tracer compounds. As eachsubstituted fluorene will possess a different mass or affinity to theseparation column, they can all be distinguishable from each other bygas chromatography mass spectrometry (GC-MS). Such a suite of tracercompounds is very useful for marking hydrocarbon liquids (e.g. fuels)from different sources and/or for marking a hydrocarbon liquid with acombination of different tracer molecules.

A method of marking a hydrocarbon fuel is thus provided, such as agasoline fuel, diesel fuel, kerosene-based fuel, liquified petroleum gasfuel, bio-diesel fuel, or bio-ethanol fuel, comprising adding a tracercompound as defined above to the hydrocarbon fuel. It is also envisagedthat the tracer compounds as described herein may be used in otherapplications, particularly other tracer applications. For example, thecompounds as described herein may be used in hydrocarbon reservoirtracing methods where the tracer compound is introduced into ahydrocarbon reservoir and then detected in fluids produced from thehydrocarbon reservoir.

Further still, there is also provided a hydrocarbon fuel, such as agasoline fuel, diesel fuel, kerosene-based fuel, liquified petroleum gasfuel, bio-diesel fuel, or bio-ethanol fuel, comprising a tracer compoundas defined above.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention and to show how thesame may be carried into effect, certain embodiments of the presentinvention will now be described by way of example only with reference tothe accompanying drawings, in which:

FIG. 1 shows the generic structure of a family of substituted fluorenetracer compounds featuring substitution at the C-2, C-7 and C-9positions;

FIG. 2 shows an example of a substituted fluorene tracercompound—2,7-di-tert-butylfluorene; FIG. 3 shows another example of asubstituted fluorene tracercompound—2,7-di-tert-butyl-9,9-dipropylfluorene;

FIG. 4 shows another example of a substituted fluorene tracercompound—2,7-di-tert-butyl-9,9-di-(2-ethylhexyl)fluorene;

FIG. 5 shows a reaction scheme for the synthesis of2,7-di-tert-butyl-9,9-di-(2-ethylhexyl)fluorene;

FIG. 6 shows the relative elution of2,7-di-tert-butyl-9,9-dipropylfluorene with components in diesel fuel;

FIG. 7 shows the relative elution of2,7-di-tert-butyl-9,9-di-(2-ethylhexyl)fluorene with components indiesel fuel;

FIG. 8 shows the relative elution of 2,7-di-tert-butylfluorene withcomponents in diesel fuel;

FIG. 9 shows the relative elution of 9,9-di-n-octylfluorene withcomponents in diesel fuel; and

FIG. 10 shows the relative elution times by GC-MS for9,9-di-n-octylfluorene, 2,7-di-tert-butylfluorene,2,7-di-tert-butyl-9,9-dipropylfluorene and2,7-di-tert-butyl-9,9-di-(2-ethylhexyl)fluorene.

DETAILED DESCRIPTION

As described in the summary section, the present specification providesa tracer compound for marking a hydrocarbon liquid is provided, thetracer compound being a substituted fluorene having a structure ofFormula I:

-   -   wherein R1 and R2 are the same or different and selected from        hydrogen, straight chain, branched or cyclic alkyl groups,        phenyl or substituted phenyl groups, benzyl or substituted        benzyl groups, or R1 and R2 form a single substituent linked        intramolecularly to each other, or R1 and R2 are ether groups        excluding acetal groups,    -   wherein R3 and R4 are the same or different and selected from        hydrogen, straight chain, branched or cyclic alkyl groups,        phenyl or substituted phenyl groups, benzyl or substituted        benzyl groups, and    -   wherein at least one of R1, R2, R3, and R4 is not hydrogen.

According to certain examples R3 and R4 are not hydrogen. In suchexamples R1 and R2 can be hydrogen. That is, substitution of thefluorene at the C-9 position is optional but can be advantageous for thereasons described previously in the summary section.

Alternatively, R1 and R2 are not hydrogen. In such examples R3 and R4can be hydrogen. That is, substitution of the fluorene on the aromaticrings is optional but can be advantageous for the reasons describedpreviously in the summary section.

According to further examples, all of R1, R2, R3, and R4 are nothydrogen. That is, the fluorene is substituted on the aromatic rings andalso at the C-9 position.

Each R group can consist of atoms selected from the group carbon,hydrogen, and oxygen. As such, for applications which specify that thetracer must only contain carbon, hydrogen, and/or oxygen atoms,embodiments of the tracer compound as described herein can fulfil thisrequirement.

Each R group can be a C₃ to C₂₀ group. The R groups can advantageouslybe straight chain, branched or cyclic alkyl groups. Particularly usefulare non-planar, branched alkyl groups such as tert-butyl, 2-ethylhexyland neo-pentyl groups.

If R1 and/or R2 are ether groups, R1 and R2 can be selected from astraight chain, branched or cyclic alkyl group, substituted phenyl, orsubstituted benzyl where each incorporates one or more oxygen atoms soas to form an ether, but where R1 and R2 do not constitute an acetal.

According to certain examples, R3 and R4 are at the C-2 and C-7positions such that the substituted fluorene tracer compound has astructure of Formula II:

Furthermore, while the preceding examples have shown a singlesubstituent on each of the aromatic rings of the core fluorenestructure, it is also envisaged that one or both of the aromatic ringsof the substituted fluorene tracer compound is substituted with one ormore further groups selected from straight chain, branched or cyclicalkyl groups, phenyl or substituted phenyl groups, benzyl or substitutedbenzyl groups.

A method of marking a hydrocarbon liquid is also provided comprisingadding a tracer compound as described herein to the hydrocarbon liquid.The resultant product is a hydrocarbon liquid, such as a gasoline ordiesel fuel, comprising the tracer compound. The hydrocarbon liquid maybe a pure compound such as hexane or octane or it may comprise a mixtureof compounds such as a distillation fraction having a particular rangeof boiling points. The hydrocarbon liquid may be intended for use as achemical, a solvent or a fuel. The hydrocarbon liquid may be abiologically derived fuel such as a bio-diesel or bio-ethanol or amixture of a biologically derived with a mineral oil derived fuel. Thetracer compounds as described herein are of particular use for markingliquid hydrocarbon fuels such as gasoline, diesel fuels, kerosene-basedfuels or liquified petroleum gas. In one particular application, alow-tax fuel such as an agricultural diesel may be marked in order todetect any subsequent sale and use for purposes such as road-vehiclefuel, which would normally be taxed more highly. In such cases unlawfuldilution or substitution of a more highly taxed fuel with the low-taxedfuel may be detected by analysis of the highly taxed fuel to determinewhether the tracer is present. Therefore, in these cases, it is highlybeneficial to use a tracer compound in the low-taxed fuel which is noteasily removed, or laundered, from the fuel to a level at which it canno longer be detected. We have found that compounds as described hereinare resistant to removal from hydrocarbon fuels by multiple knownmethods of fuel laundering.

The tracer compound is added to the hydrocarbon liquid in such an amountas to provide a concentration of the tracer compound which is detectableby readily available laboratory methods capable of identifying thetracer compound in the liquid at the concentrations used. Suitablemethods include but are not limited to gas chromatography coupled with asuitable detector such as a mass spectrometer. Typical concentrationsare within the range 1 μg/l to 10000 μg/l with the specific amountdependent on the detection method and limit of detection of theparticular tracer compound used. The tracer compound may be present at ahigher concentration than 10000 μg/l although when the product to bemarked is a high-volume commodity such as a motor-fuel, economicconsiderations usually favour lower levels of tracer compound. Thetracer compound may be supplied in the form of a concentrated dosingsolution (or master-batch) of the tracer compound in a solvent. In thiscase, the preferred solvent is a liquid which is similar to the liquidto be marked, although a different solvent, e.g. a single or mixedcomponent aliphatic or aromatic solvent may be used, provided thepresence of such a solvent can be tolerated in the hydrocarbon liquid tobe marked. A preferred solvent is solvent naphtha, optionally C10-C13low naphthalene aromatic solvent or an equivalent. The concentrateddosing solution can be added to the hydrocarbon liquid to be marked toproduce on dilution the required final concentration of the tracer inthe liquid. More than one tracer compound may be added to thehydrocarbon liquid or to the hydrocarbon fuel.

Examples of the invention as described herein generate a family ofnon-polar, non-planar molecules from a core molecule based on fluorene.These molecules are advantageous for use as tracer molecules inhydrocarbon fuels as they satisfy the following criteria: highresistance to laundering; contain only carbon and hydrogen; relativelyhigh molecular weight; relatively quick and generally distinct elutiontimes by GC-MS; non-hazardous; and a similar method of synthesis.

FIG. 1 shows the generic structure of a sub-family of substitutedfluorene tracer compounds comprising substituents R3, R4 at the C-2 andC-7 positions on the aromatic rings of the fluorene core structure withoptional substituents R1, R2 at the C-9 position. Examples of suchtracer compounds include: 2,7-di-tert-butylfluorene as shown in FIG. 2 ;2,7-di-tert-butyl-9,9-dipropylfluorene as shown in FIGS. 3 ; and2,7-di-tert-butyl-9,9-di(2-ethylhexyl)fluorene as shown in FIG. 4 .

FIG. 5 shows a reaction scheme for the synthesis of2,7-di-tert-butyl-9,9-di-(2-ethylhexyl) fluorene. The experimentalprotocol, as described below, is based on that obtained from page 151 of“Fluorene-based fluorescent markers: new insights in synthesis andapplications into labelling of nucleic acids and imaging of cellmembranes”, by Janah Shaya at Université de Nice-Sophia Antipolis, Côted'Azur. An alternative synthetic method can be found in: G. Saikia, P KIyer, J Org Chem 2010, 75, 2714-2717.

EXAMPLES Example 1

2,7-Di-tert-butylfluorene (1 g, 3.59 mmoles) was weighed directly into a50 ml round bottom flask. Potassium iodide (195 mg, 1.17 mmole, 0.3eq.), 1-bromopropane (5.3 g, 43 mmole, 12 eq.) and dimethylsulfoxide (15ml) were added. Lastly a small quantity of finely ground potassiumhydroxide (1.27 g, 22.5 mmole, 6.3 eq.) was added. The flask was fittedwith a stirrer bar and condenser and then heated over-night to 80° C. inan oil bath under air. The contents turned deep orange and a whiteprecipitate formed.

The crude reaction mix was worked up by addition to iso-octane (50 ml)and water (50 ml). The iso-octane was washed with water (2×50 ml) andthen dried over anhydrous magnesium sulfate. Evaporation under reducedpressure gave a brown oil−Yield=0.808 g (69.4%).

GC-MS analysis showed a mono-alkylated impurity (5% peak area) havingmass 320.5 amu and the di-alkylated product (95% peak area) having mass362.6 amu. The oil was purified by column chromatography followed byrecrystallisation from ethanol. The2,7-di-tert-butyl-9,9-dipropylfluorene so obtained was used in thesubsequent work.

Analysis of Example 1 in fuel

The crystallised 2,7-di-tert-butyl-9,9-dipropylfluorene (20.7 mg) wasadded to a 10 ml volumetric flask and made to the mark with iso-octane.The diluted alkyl fluorene (241 microlitre) was added to diesel fuel(250 ml) to give a tag level of 2 mg/L. The tagged diesel fuel wasanalysed by GC MS in selective ion monitoring (SIM) mode at 362 amu. Anuntagged diesel sample was also analysed in both SIM mode at 362 amu andalso in SCAN mode. The results are shown in FIG. 6 : lowertrace=untagged diesel sample in SIM mode; middle trace=tagged dieselfuel in SIM mode; upper trace=untagged diesel sample in SCAN mode. Thetagged diesel sample shows a clear signal at a retention time of 7.58min corresponding to the 2,7-di-tert-butyl-9,9-dipropylfluorene tracercompound. The chromatogram for diesel fuel not containing this tagmolecule has no background at the same mass and time as the tag moleculebut a small number of peaks in the same region. It is concluded that2,7-di-tert-butyl-fluorene can be successfully bis-alkylated at the9-position, the product is soluble in organic solvents, and itsrelatively high mass means it can readily be detected in the dieselfuel.

Example 2

2,7-Di-tert-butylfluorene (1 g, 3.59 mmoles) was weighed directly into a50 ml round bottom flask. Potassium iodide (60 mg, 0.36 mmole, 0.1 eq.)and dimethylsulfoxide (30 ml) were added. 2-ethylhexylbromide (2.78 g,14.4 mmole) was added. The reagents all dissolved. Lastly a smallquantity of finely powdered potassium hydroxide (0.806 g, 14.4 mmole)was added. The flask was fitted with a stirrer bar and condenser andthen heated to 80° C. in an oil bath under air. After a few minutesheating the colour began to yellow slightly. The reaction mix was leftover-night at room temperature during which time little further colourchange occurred.

The reaction mix was poured into water (50 ml) and left to settle. Ayellow oil separated to the surface after a few minutes. The aqueouslayer was removed, and the oil diluted with iso-octane (50 ml). Theiso-octane was washed with water (2×50 ml) and then dried over anhydrousmagnesium sulfate. Evaporation under reduced pressure gave a yellowoil−Yield=1.364 g (75.6%).

GC-MS analysis showed a mono-alkylated impurity having mass 390.7 amuand the intended di-alkylated product having mass 502.9 amu. The2,7-di-tert-butyl-9,9-(2-ethylhexyl)fluorene was used without furtherpurification as it constituted 93% of the total area by GC-MS.

Analysis of Example 2 in Fuel

The crude 2,7-di-tert-butyl-9,9-di-(2-ethylhexyl)fluorene (28.7 mg) wasadded to a 25 ml volumetric flask and made to the mark with decalin. Thediluted alkyl fluorene (871 microlitre) was added to diesel fuel 500 ml)to give a tag level of 2 mg/L. The tagged diesel fuel was analysed by GCMS in selective ion monitoring (SIM) mode at 502 amu. An untagged dieselsample was also analysed in both SIM mode at 502 amu and also in SCANmode. The results are shown in FIG. 7 : lower trace=untagged dieselsample in SIM mode; middle trace=tagged diesel fuel in SIM mode; uppertrace=untagged diesel sample in SCAN mode. The tagged diesel sampleshows a large signal at a retention time of 9.37 mins corresponding tothe 2,7-di-tert-butyl-9,9-di(2-ethylhexyl)fluorene tracer compound. Thechromatogram for diesel fuel not containing this tag molecule has nobackground at the same mass and time as the tag molecule and no peaks inthe same region. It is concluded that 2,7-di-tert-butyl-fluorene can besuccessfully bis-alkylated at the 9-position, the product is soluble inorganic solvents, and its high mass means it is readily detected indiesel fuel.

Further Examples

FIG. 8 shows the relative elution of 2,7-di-tert-butylfluorene withcomponents in diesel fuel. The tagged diesel fuel was analysed by GC MSin selective ion monitoring (SIM) mode at 263 amu. An untagged dieselsample was also analysed in both SIM mode at 263 amu and also in SCANmode. The results are shown in FIG. 8 : lower trace=untagged dieselsample in SIM mode; middle trace=tagged diesel fuel in SIM mode; uppertrace=untagged diesel sample in SCAN mode. The tagged diesel sampleshows a clear signal at a retention time of 7.80 mins corresponding tothe 2,7-di-tert-butylfluorene tracer compound. The chromatogram fordiesel fuel not containing this tag molecule has an appreciablebackground signal at the same mass and time as the tag molecule. Thebackground has approximately one quarter the area of the tag molecule.However, the 2,7-di-tert-butylfluorene can still be measured. It can beseen that although the elution time for 2,7-di-tert-butylfluorene issimilar to that of 2,7-di-tert-butyl-9,9-dipropylfluorene, the lattercompound is far easier to identify in the fuel matrix as a result of itsmolecular ion having a larger mass. The benefit of searching for atracer compound of larger molecular mass is that although there may befuel components of similar retention time, they will have a lower massand so will be ‘screened out’ of the chromatogram, making the tracercompound far easier to observe.

FIG. 9 shows the relative elution of 9,9-di-n-octylfluorene withcomponents in diesel fuel. The tagged diesel fuel was analysed by GC MSin selective ion monitoring (SIM) mode at 390 amu. An untagged dieselsample was also analysed in both SIM mode at 390 amu and also in SCANmode. The results are shown in FIG. 9 : lower trace=untagged dieselsample in SIM mode; middle trace=tagged diesel fuel in SIM mode; uppertrace=untagged diesel sample in SCAN mode. The tagged diesel sampleshows a large signal at a retention time of 9.11 mins corresponding tothe 9,9-di-n-octylfluorene tracer compound. The chromatogram for dieselfuel not containing this tag molecule has no background signal at thesame mass and time as the tag molecule. The relatively high mass of thetag molecule means it can be easily and selectively identified when itis present in the fuel.

The relative retention times of a number of fluorene derivativesanalysed by the same GC-MS method are shown in FIG. 10 . The compoundsand their respective retention times were:

-   -   2,7-di-tert-butyl-9,9-dipropylfluorene—retention time 7.58 min;    -   2,7-di-tert-butylfluorene—retention time 7.80 min;    -   9,9-di-n-octylfluorene)—retention time 9.11 min; and    -   2,7-di-tert-butyl-9,9-di-(2-ethylhexyl)fluorene)—retention time        9.37 min.

Launder Tests

A range of fuel laundering tests have been performed on individualsamples of diesel fuel containing 2,7-di-tert-butylfluorene,2,7-di-tert-butyl-9,9-dipropylfluorene,2,7-di-tert-butyl-9,9-di(2-ethylhexyl)fluorene, and9,9-di-n-octylfluorene.

Samples of the tagged fuel were subjected to a series of launder testswhere the fuel was subjected to commonly used laundering reagents. Inthe procedure that follows, a sample of tagged fuel that has beensubjected to laundering is referred to as ‘laundered fuel’; a sample oftagged fuel that had not been subjected to laundering is referred to as‘tagged reference’. In order to assess the degree of removal of thetaggant by the laundering reagent, the concentration of the taggant inlaundered fuel was compared after a particular launder test with theconcentration of the taggant in a sample of the same fuel which had notbeen subjected to any launder test. A typical GC sequence includedtagged reference, untagged fuel, samples of laundered fuel, taggedreference and finally untagged fuel. Reference samples were run at thebeginning and end of any GC sequence to help eliminate instrument driftover the course of the sequence.

Analytical Conditions

Injection size: 1 μL.

Solvent wash: 2×10 μL solvent A, 2×10 μL solvent B.

Sample rinse: 2×10 μL solvent A, 2×10 μL solvent B.

Inlet: split; temperature: 270° C.; pressure 11.8 psi; spilt ratio 40:1;spilt flow 96.6 ml/min; total flow 100.9 ml/min; carrier gas helium.

Column: Agilent HP-5MS; 30 m×0.25 mm i.d.×0.25 μm; stationary phase (5%phenyl)-methylpolysiloxane.

Mode: constant flow.

Oven temperature: 80° C. for 0.5 min, 25° C./min up to 325° C., hold1.70 min.

Mass Spectrometer Conditions

Transfer line temperature: 280° C.

Quadrupole temperature: 150° C.

Source temperature: 230° C.

Operating mode: Selective Ion Monitoring (SIM) with ions as appropriatefor the molecule being analysed.

Dwell time: 100 msec.

All those launder tests involving a washing procedure were carried outin sealed brown glass bottles to minimise evaporation over the four-hourstirring period. All launder tests involving stirring were allowed toseparate before sampling. The fuel layer from any launder testcontaining an aqueous reagent was separated into a scintillation vialwhere it was dried over anhydrous magnesium sulfate or potassiumcarbonate before being filtered through a cotton wool plug and finallytransferred to a GC vial. All tests involving the passage of fuelthrough a column of solid adsorbent were carried out by applying reducedpressure to the outlet of the column rather than a positive pressure tothe mouth of the column. This was achieved by fitting the column outletvia a close-fitting seal to a receptacle, such as a Buchner flask andcollecting the liquid that elutes from the column into the flask byattaching a vacuum pump to the side arm. Fuels containing obviousparticulate matter were filtered before dispensing into a GC vial. Thefuel from all other launder tests was sampled into GC vials withoutfurther clean-up.

Launder tests included the following:

-   -   Hydrochloric acid wash—hydrochloric acid (25 mL, 10% w/w) was        mixed with tagged fuel (25 ml) and stirred for four hours at        room temperature.    -   Sulfuric acid wash—Sulfuric acid (25 ml, 10% w/w) was mixed with        tagged fuel (25 ml) and stirred for four hours at room        temperature.    -   Sodium hydroxide wash—Sodium hydroxide solution (25 ml, 10M) was        mixed with tagged fuel (25 ml) and stirred for four hours at        room temperature.    -   Methanolic potassium hydroxide wash—potassium hydroxide (1 M) in        methanol (25 ml) was mixed with tagged fuel (25 ml) and stirred        for four hours at room temperature.    -   Methanol wash—methanol (25 ml) was mixed with tagged fuel        (25 ml) and stirred for four hours at room temperature.    -   Acetonitrile wash—acetonitrile (25 ml) was mixed with tagged        fuel (25 ml) and stirred for four hours at room temperature.    -   Hydrogen peroxide wash—hydrogen peroxide solution (27%, aqueous,        25 ml) was mixed with tagged fuel (25 ml) and stirred for four        hours at room temperature.    -   60° C. stir—tagged fuel (50 ml) was placed in a beaker and        stirred at 60° C. for four hours.    -   Aeration—tagged fuel (50 ml) in a brown glass bottle was bubbled        with air at about 200 ml/minute. The fuel was analysed        periodically.    -   UV treatment—two samples of fuel (25 ml) in clear glass        scintillation vials were stored under a bench top UV light. One        vial was open to the light; the other was sealed and laid on its        side under the light. The samples were analysed periodically.    -   Activated charcoal stir—activated charcoal (2.5 g, Norit SX plus        F Cat, p/no. 901933, Sigma Aldrich) was mixed with tagged fuel        (50 ml) and stirred for four hours at room temperature.    -   Fuller earth stir—Fullers earth (2.5 g) was mixed with tagged        fuel (50 ml) and stirred for four hours at room temperature.    -   Activated charcoal columns—a 10 cm glass column with 1 cm        internal diameter was packed with activated charcoal (Norit        RBAA-3 rod). Tagged fuel (50 ml) was passed through the column.        Two repeat passes of the fuel through the coloumn were carried        out using fresh activated charcoal each time. A similar        procedure was carried out using columns containing Fullers earth        (100-200 mesh, p/no. F200, Sigma Aldrich), sepiolite (no        supplier details) and Davisil silica (grade 710, 50-76 Å, p/no.        236756, Sigma Aldrich).

Results of the launder tests in diesel fuel are summarized in the tablesbelow indicating fuel type, launder test, and amount of tag molecule ortracer remaining after the test in terms of a percentage of the initialconcentration of the tracer/tag in the fuel.

Tag Amount level remaining Tag (mg/l) Fuel Launder test after test (%)2,7-di-tert-butyl- 2 Diesel Hydrochloric acid wash 95.7 fluorene2,7-di-tert-butyl- 2 Diesel Sulfuric acid wash 96.9 fluorene2,7-di-tert-butyl- 2 Diesel Sodium hydroxide wash 98.0 fluorene2,7-di-tert-butyl- 2 Diesel Activated charcoal 109.5 fluorene column -1^(st) pass 2,7-di-tert-butyl- 2 Diesel Activated charcoal 107.7fluorene column - 2^(nd) pass 2,7-di-tert-butyl- 2 Diesel Activatedcharcoal 92.6 fluorene column - 3^(rd) pass

Tracer Amount level remaining Tracer (mg/l) Fuel Launder test after test(%) 9,9-di-n- 2 Diesel Hydrochloric acid wash 95.6 actylfluorene9,9-di-n- 2 Diesel Sulfuric acid wash 100.8 actylfluorene 9,9-di-n- 2Diesel Sodium hydroxide wash 94.0 actylfluorene 9,9-di-n- 2 DieselActivated charcoal 106.5 actylfluorene column - 1^(st) pass 9,9-di-n- 2Diesel Activated charcoal 93.9 actylfluorene column - 2^(nd) pass9,9-di-n- 2 Diesel Activated charcoal 98.1 actylfluorene column - 3^(rd)pass

Tag Amount level remaining Tag (mg/l) Fuel Launder test after test (%)2,7-di-tert-butyl- 2 Diesel Hydrochloric acid 95.8 9,9-di(2- washethylhexyl)fluorene 2,7-di-tert-butyl- 2 Diesel Sulfuric acid wash 93.39,9-di(2- ethylhexyl)fluorene 2,7-di-tert-butyl- 2 Diesel Sodiumhydroxide 97.8 9,9-di(2- wash ethylhexyl)fluorene 2,7-di-tert-butyl- 2Diesel Activated charcoal 97.0 9,9-di(2- column - 1^(st) passethylhexyl)fluorene 2,7-di-tert-butyl- 2 Diesel Activated charcoal 97.09,9-di(2- column - 2^(nd) pass ethylhexyl)fluorene 2,7-di-tert-butyl- 2Diesel Activated charcoal 99.8 9,9-di(2- column - 3^(rd) passethylhexyl)fluorene

Amount Tag remaining level after test Tag (mg/l) Fuel Launder test (%)2,7-di-tert-butyl-9,9- 2 Diesel Hydrochloric acid 96.4 dipropylfluorenewash 2,7-di-tert-butyl-9,9- 2 Diesel Sulfuric acid wash 108.0dipropylfluorene 2,7-di-tert-butyl-9,9- 2 Diesel Sodium hydroxide 105.2dipropylfluorene wash 2,7-di-tert-butyl-9,9- 2 Diesel Methanolicpotassium 99.3 dipropylfluorene hydroxide wash 2,7-di-tert-butyl-9,9- 2Diesel Methanol wash 108.8 dipropylfluorene 2,7-di-tert-butyl-9,9- 2Diesel Acetonitrile wash 90.6 dipropylfluorene 2,7-di-tert-butyl-9,9- 2Diesel Hydrogen peroxide 94.2 dipropylfluorene wash2,7-di-tert-butyl-9,9- 2 Diesel Stir at 60° C. 117.4 dipropylfluorene2,7-di-tert-butyl-9,9- 2 Diesel Aeration - 24 hour 99.4 dipropylfluorene2,7-di-tert-butyl-9,9- 2 Diesel Aeration - 48 hour 106.2dipropylfluorene 2,7-di-tert-butyl-9,9- 2 Diesel UV open - 24 hour 100.0dipropylfluorene 2,7-di-tert-butyl-9,9- 2 Diesel UV open - 48 hour 109.8dipropylfluorene 2,7-di-tert-butyl-9,9- 2 Diesel UV open - 168 hour104.9 dipropylfluorene 2,7-di-tert-butyl-9,9- 2 Diesel UV closed - 24hour 98.7 dipropylfluorene 2,7-di-tert-butyl-9,9- 2 Diesel UV closed -48 hour 98.3 dipropylfluorene 2,7-di-tert-butyl-9,9- 2 Diesel UVclosed - 168 hour 109.7 dipropylfluorene 2,7-di-tert-butyl-9,9- 2 DieselActivated charcoal stir 104.7 dipropylfluorene 2,7-di-tert-butyl-9,9- 2Diesel Fullers earth - stir 101.4 dipropylfluorene2,7-di-tert-butyl-9,9- 2 Diesel Activated charcoal 105.4dipropylfluorene column - 1^(st) pass 2,7-di-tert-butyl-9,9- 2 DieselActivated charcoal 103.8 dipropylfluorene column - 2^(nd) pass2,7-di-tert-butyl-9,9- 2 Diesel Activated charcoal 104.2dipropylfluorene column - 3^(rd) pass 2,7-di-tert-butyl-9,9- 2 DieselAlumina column - 1^(st) 101.8 dipropylfluorene pass2,7-di-tert-butyl-9,9- 2 Diesel Alumina column - 2^(nd) 105.7dipropylfluorene pass 2,7-di-tert-butyl-9,9- 2 Diesel Alumina column -3^(rd) 104.3 dipropylfluorene pass 2,7-di-tert-butyl-9,9- 2 DieselFullers earth column - 98.1 dipropylfluorene 1^(st) pass2,7-di-tert-butyl-9,9- 2 Diesel Fullers earth column - 107.6dipropylfluorene 2^(nd) pass 2,7-di-tert-butyl-9,9- 2 Diesel Fullersearth column - 91.4 dipropylfluorene 3^(rd) pass 2,7-di-tert-butyl-9,9-2 Diesel Sepiolite column - 1^(st) 104.7 dipropylfluorene pass2,7-di-tert-butyl-9,9- 2 Diesel Sepiolite column - 2^(nd) 112.3dipropylfluorene pass 2,7-di-tert-butyl-9,9- 2 Diesel Sepiolite column -3^(rd) 106.7 dipropylfluorene pass 2,7-di-tert-butyl-9,9- 2 DieselSilica column - 1^(st) pass 103.3 dipropylfluorene2,7-di-tert-butyl-9,9- 2 Diesel Silica column - 2^(nd) pass 115.6dipropylfluorene 2,7-di-tert-butyl-9,9- 2 Diesel Silica column - 3^(rd)pass 103.3 dipropylfluorene

As can be seen for the results table, even when reference samples wererun at the beginning and end of the GC sequence to help eliminateinstrument drift over the course of the sequence, some of the resultsindicate tracer concentrations above 100% after the launder test. Thiscould arise from an imperfect correction of instrument drift or from theremoval of components from the fuel by the laundering process leading toan increase in tracer molecule concentration. That said, it should benoted that no problem or interference was experienced in analysing byGC-MS for the taggant molecules.

For the launder tests, when analysing for:2,7-di-tert-butyl-9,9-dipropylfluorene the mass spectrometer was set todetect m/e=362 amu; when analysing for2,7-di-tert-butyl-9,9-di(2-ethylhexyl)fluorene the mass spectrometer wasset to detect m/e=502; when analysing for 2,7-di-tert-butyl fluorene themass spectrometer was set to detect m/e=263 amu; and when analysing for9,9-di-n-octyl-fluorene the mass spectrometer was set to detect m/e=390amu. Results show that all of the fluorene derivatives can be readilymeasured and they are all resistant to the laundering tests in dieselfuel. It was found that the mass of the molecular ion is often the mostconvenient mass to analyse however, when analysing for2,7-di-tert-butylfluorene the ion at 263 amu was significantly moreintense.

While this invention has been particularly shown and described withreference to certain embodiments, it will be understood to those skilledin the art that various changes in form and detail may be made withoutdeparting from the scope of the invention as defined by the appendedclaims.

1-14. (canceled)
 15. A method of marking a hydrocarbon fuel, the methodcomprising adding to said hydrocarbon fuel a tracer compound for markingthe hydrocarbon fuel, the tracer compound being a substituted fluorenehaving a structure of Formula I:

wherein R1 and R2 are the same or different and selected from hydrogen,straight chain, branched or cyclic alkyl groups, phenyl or substitutedphenyl groups, benzyl or substituted benzyl groups, or R1 and R2 form asingle substituent linked intramolecularly to each other, or R1 and R2are ether groups excluding acetal groups, wherein R3 and R4 are the sameor different and selected from hydrogen, straight chain, branched orcyclic alkyl groups, phenyl or substituted phenyl groups, benzyl orsubstituted benzyl groups, wherein at least one of R1, R2, R3, and R4 isnot hydrogen, and wherein the hydrocarbon fuel is a gasoline fuel, adiesel fuel, a kerosene-based fuel, a liquified petroleum gas fuel, abio-diesel fuel, or a bio-ethanol fuel.
 16. The method according toclaim 15 wherein R3 and R4 are not hydrogen.
 17. The method according toclaim 15 wherein R1 and R2 are not hydrogen.
 18. The method according toclaim 15 wherein R1, R2, R3, and R4 are selected such that thesubstituted fluorene tracer compound consists only of atoms selectedfrom the group carbon, hydrogen, and oxygen.
 19. The method according toclaim 15 wherein R1 and R2 and/or R3 and R4 are selected from the sameor different C₃ to C₂₀ group.
 20. The method according to claim 15wherein R1 and R2 and/or R3 and R4 are selected from the same ordifferent, straight chain, branched or cyclic alkyl groups.
 21. Themethod according to claim 20 wherein R1 and R2 and/or R3 and R4 areselected from the same or different branched or cyclic alkyl groups. 22.The method according to claim 21 wherein R1 and R2 and/or R3 and R4 areselected from the same or different branched alkyl groups.
 23. A methodaccording to claim 22 wherein R3 and R4 are tert-butyl groups.
 24. Amethod according to claim 15 wherein R3 and R4 are at the C-2 and C-7positions of the aromatic rings of the substituted fluorene tracercompound such that the substituted fluorene tracer compound has astructure of Formula II:


25. A hydrocarbon fuel comprising the tracer compound as defined inclaim 15, wherein the hydrocarbon fuel is a diesel fuel, a gasolinefuel, a kerosene-based fuel, a liquified petroleum gas fuel, abio-diesel fuel, or a bio-ethanol fuel in which the tracer compound isdisposed.
 26. Use of a compound as a tracer compound for marking ahydrocarbon fuel, the compound being a substituted fluorene having astructure of Formula I:

wherein R1 and R2 are the same or different and selected from hydrogen,straight chain, branched or cyclic alkyl groups, phenyl or substitutedphenyl groups, benzyl or substituted benzyl groups, or R1 and R2 form asingle substituent linked intramolecularly to each other, or R1 and R2are ether groups excluding acetal groups, wherein R3 and R4 are the sameor different and selected from hydrogen, straight chain, branched orcyclic alkyl groups, phenyl or substituted phenyl groups, benzyl orsubstituted benzyl groups, wherein at least one of R1, R2, R3, and R4 isnot hydrogen, and wherein the hydrocarbon fuel is a diesel fuel, agasoline fuel, a kerosene-based fuel, a liquified petroleum gas fuel, abio-diesel fuel, or a bio-ethanol fuel.